In the operation of components, especially components of gas turbines, at high temperatures, their free surfaces are exposed to strongly corrosive and oxidative conditions. In the application in gas turbines, such components can, for example, consist of a super-alloy on a nickel basis or a cobalt basis. The components are provided with coatings for protection against corrosion, oxidation or also erosion. PtAl coatings are preferred, with which an especially good corrosion protection and/or oxidation protection can be realized.
The EP 0 784 104 B1 discloses a PtAl coating for gas turbine components as well as a method for production of such a coating. According to the method described there, a PtAl coating is produced on a substrate in that a platinum layer is deposited on a substrate surface, whereby a diffusing of platinum from the platinum layer into the substrate surface is carried out after the deposition of the platinum layer. After the deposition of the platinum layer and the in-diffusion of the platinum, the thusly coated substrate is alitized or aluminized, i.e. coated with aluminum, whereby the aluminum is preferably diffused into the substrate surface.
The deposition of platinum onto the substrate surface before the aluminizing of the substrate preferably occurs in a galvanic manner. The present invention relates to details of a method for production of a corrosion resistant and/or oxidation resistant coating on a substrate, which relate to the galvanic deposition of a metal of the platinum group, in particular of platinum and/or palladium, or an alloy based on at least one metal of the platinum group. Thus, it is of significant importance for the quality of the corrosion resistant and/or oxidation resistant coating, that a uniformly defined deposition of particularly platinum is realized in a galvanic manner, in order to thereby realize a uniform thickness of a platinum coating. Thus, for example, the coating thickness may not undershoot or fall below a minimum value of the coating thickness of approximately 1 μm, because this would give rise to an inadequate hot gas resistance and a local rapid failure of the coating. On the other hand, layer thicknesses of 8 to 15 μm may not be exceeded, because hereby on the one hand valuable precious metal would be wasted and on the other hand the characteristics of the coating would be made worse. A further problem of galvanic deposition of particularly platinum on a substrate exists when the platinum, for example, is to be deposited onto structural components with a complex three-dimensional configuration. Such substrates with a complex three-dimensional contour, are, for example, gas turbine vanes or blades, because these on the one hand are strongly unsymmetrical, and on the other hand comprise edges, corners and surfaces having points as well as hollow spaces and undercuts. A uniformly defined deposition of platinum on substrates with a complex three-dimensional contour can only be inadequately realized with the methods known from the state of the art for the galvanic deposition of platinum.